1. Field of the Invention
The present invention relates to equipment for making resistivity measurements while drilling a wellbore, and in particular, the invention relates to multipole antennas.
2. Description of the Related Art
Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, reasonably determine the petrophysical properties of the formation and the fluids therein.
The physical principles of electromagnetic induction resistivity well logging are described, for example, in H. G. Doll, Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil-Based Mud, Journal of Petroleum Technology, vol. 1, p. 148, Society of Petroleum Engineers, Richardson, Tex. (1949). Many improvements and modifications to electromagnetic induction resistivity instruments have been devised since publication of the Doll reference, supra. Examples of such modifications and improvements can be found, for example, in U.S. Pat. No. 4,837,517 issued to Barber; U.S. Pat. No. 5,157,605 issued to Chandler et al.; and U.S. Pat. No. 5,452,761 issued to Beard et al.
A typical electrical resistivity-measuring instrument is an electromagnetic induction military well logging instrument such as described in U.S. Pat. No. 5,452,761, issued to Beard et al. The induction logging instrument described in the Beard '761 patent includes a number of receiver coils spaced at various axial distances from a transmitter coil. Alternating current is passed through the transmitter coils, which induces alternating electromagnetic fields in the earth formations. Voltages, or measurements, are induced in the receiver coils as a result of electromagnetic induction phenomena related to the alternating electromagnetic fields. A continuous record of the voltages form curves, which are also referred to as induction logs. The induction instruments that are composed of multiple sets of receiver coils are referred to as multi-array induction instruments. Every set of receiver coils together with the transmitter is named as a subarray. Hence, a multi-array induction consists of numerous subarrays and acquires measurements with all the subarrays.
Logging-while-drilling resistivity tools employ loop antennas to transmit and receive electromagnetic signals into and from surrounding formations, respectively. These signals provide for determination of resistivity and other electromagnetic properties of the formations. The loop antennas can have magnetic moments pointing parallel or transverse to an axis for the tool (or in any other direction). Such antennas are usually called monopole antennas because they have unidirectional magnetic moments. However, for certain applications, multipole antennas are needed. A multipole antenna can be a dipole, a quadrupole, etc.
For instance, a dipole antenna has the capability of providing the azimuthal direction information of a remote bed relative to the wellbore (Minerbo et al., U.S. Pat. No. 6,509,738). Conceptually, a dipole antenna consists of two spaced apart monopoles with one pointing to one direction and the other to the opposite direction. A quadrupole antenna consists of two spaced apart dipoles. The two dipoles point to the opposite direction.
What are needed are techniques for providing multipole antennae for conducting logging while drilling.